1. Field of the Invention
The present invention pertains generally to the field of external combustion engines such as Stirling cycle engines and is more particularly directed to a regenerative displacer and associated heat exchanger arrangement for subjecting the working fluid in such an engine to a thermodynamic cycle from which is derived a work output.
2. State of the Prior Art
In an external combustion closed cycle engine a sealed working fluid, usually a gas, is alternately displaced between a hot end and a cold end of a displacer cavity by a reciprocating displacer body such that it undergoes successive volumetric expansion and contraction cycles which drive a work piston or otherwise produce a work output.
Many different schemes have been devised for transferring heat to the working fluid during one portion of the displacer stroke and extracting heat from the fluid during another portion of the displacer stroke. Most commonly, the working fluid is passed through a heat exchanger conduit where heat is transferred by conduction between the working fluid and a heat exchanger medium. It is also conventional to provide a regenerator device in the fluid flow path between the hot and cold ends for storing heat as the working fluid passes through the cooling portion of the thermodynamic cycle.
In a conventional heat exchanger all of the working fluid displaced during each stroke of the displacer passes through a single heat exchanger conduit which must be relatively long in order to absorb sufficient heat from all the displaced fluid. As the flow of working fluid through the heat exchanger must necessarily be restricted in order to maintain good heat exchanging contact with the operative surfaces of the heat exchanger, a relatively large drop in working fluid pressure occurs across the heat exchanger. This pressure drop is particularly significant at the cold end of the displacer cavity where gas density is greatest and significantly reduces the overall efficiency of the engine. Various attempts have been made to minimize this pressure drop, such as the design illustrated in "Principles and Applications of Stirling Engines" Colin West, 1986, page 212. In this arrangement, a permeable heater is mounted on the displacer and is heated by conduction through a thin gas layer by a hot head at one end of the displacer bore. As the displacer moves from the cold end to the hot end, the working fluid passes through the porous heater and a regenerator also on the displacer, and is cooled by contact with a liquid cooled working piston. This engine, however, makes use of a very short displacer stroke and relies on large surface areas for heat transfer, in spite of which the work output achieved is minimal in relation to the engine's physical size and weight.
A continuing need exists for improved, more efficient displacer arrangements for Stirling cycle and similar engines.